1. Field of the Invention
The present invention relates to an electrically conductive porous body obtained by giving electrical conductivity onto the surface of the framework of a plastic porous body having a continuous-pore structure, to a metallic porous body produced by using the conductive porous body as an intermediate material, which metallic porous body is especially suitable for a plate used in batteries such as alkaline secondary batteries; and to a battery plate produced by using the metallic porous body.
2. Description of the Background Art
Alkaline secondary batteries have been widely used as a power source for various devices because they are highly reliable and can be reduced in size and weight. There are a variety of sizes from small types for portable devices to large types for industrial or large-scale equipment.
There are many types of alkaline secondary batteries have in terms of the combination of the positive and negative electrodes. While most cases, a nickel electrode is used as the positive electrode, various negative electrodes are used, such as, a cadmium electrode, a zinc electrode, an iron electrode, a hydrogen electrode, and so on. Of these, a cadmium electrode is the most common. Yet, a hydrogen electrode having a hydrogen-absorbing alloy as the active material has been the focus of attention with regard to capacity increase and pollution decrease.
Among the foregoing electrodes, the so-called xe2x80x9cpocket typexe2x80x9d nickel electrode was conventionally used. However, a new type of nickel electrode has been in popular use in recent years. The new type is produced by filling large quantities of particles of an active material for the positive electrode, such as nickel hydroxide, into the pores of a porous current-collecting plate made of a conductive material such as nickel. This type enables a battery to be hermetically sealed and can further improve the battery properties compared to the pocket type. Also, a cadmium electrode and a hydrogen electrode are produced by filling large quantities of active materials into the pores of a porous plate for the negative electrode, such as cadmium or hydrogen-absorbing alloy.
Formerly a sintered body of a nickel powder was used as a porous plate to be filled with an active material. However, a new type of metallic porous body has been increasingly used as the porous plate in recent years. The new type is produced by using a plastic porous body (a polyurethane foam, for example) that has a continuous-pore structure with high porosity as the core, because this type allows the filling of greater quantities of active materials compared to the sintered body, and is suitable for increasing the battery capacity.
Such a metallic porous body is usually produced by the following method:
First, conductivity is given to the plastic core by either of the following methods:
(1) The surface of the framework of the plastic core is treated with a catalyst such as palladium chloride. Next, conductivity is given onto the treated surface by electroless plating such as electroless nickel plating.
(2) A mixed-binder solution containing conductive carbon particles such as graphite is applied to the surface of the framework of the plastic core. Next, the solution is dried to complete the process of giving conductivity to the plastic core.
Second, a continuous metal-plated layer (a nickel-plated layer, for example) is formed on the surface of the conductive framework of the plastic core by electroplating with a metal (nickel, for example) with the conductive core (the conductive porous body) serving as the cathode. Finally, if necessary, the core is removed by heat treatment.
The electroless plating described in the method (1) above, however, is costly because it uses palladium, which is a noble metal. Moreover, if the palladium is admitted into the electroplating liquid, a treating liquid for the succeeding process, a rapid reducing reaction of nickel ions represented as xe2x80x9cnickel-dust formationxe2x80x9d occurs, consuming nearly all the nickel in the plating liquid, and which prevents further use of the liquid as a plating liquid. The term xe2x80x9cnickel-dust formationxe2x80x9d is used in the present invention to mean a phenomenon that nickel deposits onto palladium particles, being suspended in an electroplating liquid.
It is difficult to handle the conductive porous body produced by the method (1) above. The conductive porous body tends to increase its electrical resistance significantly when undergoing deformation during the following processes:
(a) In a continuous production system where a metallic porous body is produced continuously from the core through the conductive porous body, when the conductive porous body is supplied to the electroplating process, a process following the electroless plating, the conductive porous body is drawn longitudinally by bending or tension.
(b) When the required quantities of conductive porous bodies are grouped as a batch to be supplied to the subsequent electroplating process, the conductive porous body is wound into the form of a roll or hoop.
(c) After the required quantities of the cores wound into the form of a roll or hoop as a batch are treated in the process for obtaining conductivity, when the conductive porous body thus produced is supplied to the subsequent continuous-electroplating process, the conductive porous body is unwound from the form of a roll or hoop.
If the conductive porous body increases its electrical resistance significantly, this increase reduces the growth rate of the metal-plated layer, such as a nickel-plated layer, in the electroplating process. This rate reduction may reduce the productivity or production efficiency of the metallic porous body.
The conductive layer formed on the surface of the framework of a core by electroless plating is an extremely thin, continuous metallic film. Electroless nickel plating, for instance, produces a metallic film as thin as 0.1 xcexcm or so. The metallic film cracks or folds easily when the conductive porous body undergoes deformation caused by the bending, drawing winding, or unwinding as described above. The cracks or folds reduce the conductivity of the conductive porous body, increasing the electrical resistance significantly as mentioned above. The result is the reduction in the growth rate of the metal-plated layer, such as a nickel-plated layer, in the electroplating process.
In order to reduce the tension applied to, the conductive porous body, for instance, a decrease in the feeding speed and an increase in the radius of curvature in bending and winding have been discussed. These solutions, however, pose new problems such as the reduction in the productivity or production efficiency of the metallic porous body and the necessity for a greater area of space for the production equipment or the facilities for material handling or storing.
Electroless nickel plating, the most popular electroless plating described in the method (1) above, has used sodium hypophosphite (NaH2PO2xc2x7H2O) or sodium boron hydride (NaBH4) as a reducing agent to deposit nickel ions in the plating liquid as a metal. Consequently, the conductive layer formed on the surface of the framework of the core is inevitably made of a nickel-phosphorus alloy or a nickel-boron alloy each of which contains several percentages of phosphorus or boron derived from the reducing agent as an impurity.
In this case, when the core is removed by heat treatment after the metal-plated layer, such as a nickel-plated layer, is formed on the conductive layer by electroplating, the foregoing impurity such as phosphorus diffuses into the metal-plated layer, increasing the electrical resistance of the metallic porous body produced. As a result, a battery having a plate made with such a metallic porous body may suffer a reduction in charging and discharging efficiency or may undergo deterioration in charging and discharging properties caused by the dissolution of the phosphorus into the electrolyte after prolonged and repeated charging and discharging.
Similarly, the conductive porous body produced by applying carbon particles on the surface of the framework of the core as described in the method (2) above, also, suffers crazing of the binder resin owing to the drawing operation in particular, increasing the electrical resistance considerably as with the conductive porous body produced by the method (1). Furthermore, the conductive porous body produced by the method (2) inherently has high electrical resistance, because carbon itself has considerably higher resistivity than metal and carbon particles are adhered onto one another with the help of a binder that has no or poor conductivity.
Consequently, the conductive porous body produced by the method (2), also, has problems of a significantly low growing rate of a metal-plated layer in the electroplating process and low productivity and production efficiency of the metallic porous body.
An object of the present invention is to offer a conductive porous body that:
(a) has a conductive layer made of nickel whose impurities are reduced to a minimum;
(b) provides a high growing rate of a metallic layer in the electroplating process, is easy to handle, maintains a practical high growing rate even when it undergoes a large magnitude of deformation, and hence is excellent in productivity and production efficiency .for producing a metallic porous body; and
(c) is capable of producing an excellent metallic porous body that is low in electrical resistance.
Another object of the present invention is to offer a metallic porous body that is low in electrical resistance and enables an increase in efficiency of charge and discharge when used as a plate for batteries.
Yet another object of the present invention is to offer a battery plate that has high efficiency of charge and discharge.
In order to solve the above-described problems, the present inventors made intensive studies to find a nickel-ion-reducing substance that:
(a) has a reducing potential sufficient for reducing nickel ions;
(b)) has an extremely large ionization tendency and hence cannot be reduced by metals in an aqueous solution, so that it does not contaminate a nickel layer as an impurity metal by depositing during the electroplating process; and
(c) can be re-used by restoring the original oxidation number easily even if it is oxidized by acting as a reducing agent.
The present inventors also earnestly investigated a treating method that enables high bonding strength of the foregoing conductive particles onto the surface of the framework of a core, a plastic porous body.
The present inventors obtained the following various findings as a result
[1] First, the present inventors found that in order to form a conductive layer composed of nickel particles on the surface of the framework of a plastic porous body, it is desirable to use as a reducing agent a mixture of a specific complexing agent that increases the reducing potential of titanium ions and a titanium-chloride solution containing titanium(III) chloride.
Whereas a potential difference of 0.257 V is needed to reduce a divalent nickel ion to metal nickel, a potential difference no more than 0.04 V is needed to oxidize a trivalent titanium ion included in titanium(III) chloride to a quadrivalent ion. However, the reaction of a trivalent titanium ion with a specific complexing agent increases the potential difference between trivalence and quadrivalence in titanium ions. For instance, a complexing reaction of a trivalent titanium ion with a citric acid produces a potential difference between trivalence and quadrivalence as large as more than 1 V at pH 9.0.
Although a reducing potential more than 1 V can be obtained by using sodium hypophosphite or sodium boron hydride either of which has been used as a reducing agent for nickel, sodium hypophosphite or sodium boron hydride alone cannot reduce nickel, because nickel ions in the aqueous solution are hydrated to become stable complex ions called aquo complexes. Therefore, the conventional electroless nickel plating decomposes nickel-aquo complexes through the adsorption on the surface of a palladium catalyst to produce bare nickel ions so that the reducing reaction to metal nickel is realized. Incidentally, the present inventors found no published techniques that deposit nickel directly from the aqueous solution without using a precious metal such as palladium.
On the other hand, the present invention uses a reducing agent that mixes a complexing agent such as citric acid with a titanium-chloride solution containing titanium(III) chloride. It also uses nickel(II) sulfate as a source of nickel. The nickel(II) sulfate is caused to react with ammonia in an aqueous solution to produce a nickel-ammonium aquo complex. This method of the present invention enables the reducing reaction of nickel ions and accompanying deposition of nickel particles without using an expensive, precious-metal catalyst such as palladium. According to the present invention, a significantly high-cost treating method is not required, and there is no possibility of admitting a palladium catalyst into an electroplating liquid, so that the possibility of the occurrence of the above-described xe2x80x9cnickel-dust formationxe2x80x9d can be eliminated. Incidentally, the present invention does not exclude the use of palladium aggressively; a palladium catalyst may be used simultaneously because of industrial reasons such as an increase in production rate.
[2] It is known that the above-mentioned titanium chloride belongs to the Ziegler-Natta catalyst as with alkylaluminum and acts as a catalyst for isometric polymerization of olefins. (To be more specific, titanium chloride forms a transition state by acting on xcfx80-electron clouds in olefins.) Based on this knowledge, the present inventors found that the adhesion of nickel onto a plastic porous body can be secured by causing the porous body to adsorb titanium chloride previously or concurrently with the reducing reaction of nickel.
[3] In addition, titanium is an element that has the third highest ionization tendency after beryllium and magnesium in alkaline earth metals, and usually, titanium ions in an aqueous solution cannot be reduced to metal titanium. Therefore, metal titanium does not contaminate a nickel layer depositing on the surface of the framework of a plastic porous body after being reduced during the electroless plating process.
[4] Sodium hypophosphite and sodium boron hydride, which have been used as a reducing agent, transform into a substance that cannot be re-used after the electroless plating process. On the other hand, the quadrivallent titanium produced during the electroless plating process using a reducing agent containing the foregoing titanium compound can be restored to the original trivalent titanium by the reduction at the cathode in the electrolysis where the aniode and cathode are separated by an ion-exchange membrane in a hydrochloric acidic aqueous solution. In short, the reducing agent can be re-used by restoring its oxidation number to the original number.
[5] A catalyst is not required basically in forming a nickel layer by electroless plating when:
(a) a reducing agent is prepared by mixing a titanium-chloride solution containing titanium(III) chloride and a specific complexing agent that increases the reducing potential of titanium ions;
(b) nickel(II) sulfate is used as a source of nickel; and
(c) the nickel(II) sulfate is caused to react with ammonia in an aqueous solution to produce a nickel-ammonium aquo complex.
Furthermore, the present inventors confirmed that there is no self-catalytic reaction. Hence, nickel particles that deposited earlier in the electroless plating process onto the surface of the framework of a plastic porous body remain as particles with no growth.
[6] A conductive layer comprising nickel particles forms a thin passive coating caused by the characteristic of nickel itself, and hence is stable against water and oxygen. As a result, its surface is hardly oxidized, and it maintains high conductivity (i.e., low resistance) at all times. Such a conductive layer, when the deposited quantity of nickel particles is relatively small, increases its electrical resistance at a dry condition owing to poor electrical conduction between the particles. Nonetheless, the conductive layer can form a metal-plated layer with a high growing rate in the subsequent electroplating process. This is attributable to the conductive layer, when immersed in an electroplating bath, the electrical conduction between the particles being maintained through the plating liquid that has filled the minute gaps between the particles, reducing the electrical resistance. Consequently, a metallic porous body can be produced with good productivity and high production efficiency.
It is surprising that the foregoing high growth rate hardly changes even after the conductive porous body undergoes various types of deformation as mentioned earlier. This is attributed to the deformation scarcely affecting the structure of the conductive layer and the mechanism of electrical conduction under the immersed condition in an electroplating bath.
Subsequently, the present inventors made further studies on the microscopic structure of a conductive layer made of nickel on the basis of the findings described in [1] to [6] above, and found that a conductive layer can exhibit the foregoing excellent performance when it is composed of a collection of nickel particles, thus completing the present invention.
To be more specific, in order to solve the above-mentioned technical problems, the conductive porous body of the present invention has, on the surface of the framework of a plastic porous body with a continuous-pore structure, a conductive layer composed of nickel particles caused to deposit from an aqueous solution dissolving nickel ions by virtue of a reducing agent containing titanium compounds.
In the present invention, it is desirable that the reducing agent be a mixture of titanium(III) chloride and citric acid. It is also desirable that the nickel ions be derived from nickel(II) sulfate, nickel(II) chloride, nickel carbonate, or nickel alloy. It is preferable that the nickel ions be nickel-ammonium aquo complexes produced by a reaction of nickel(II) sulfate (a starting material of nickel ions) and ammonia in an aqueous solution.
It is desirable that the conductive layer in the conductive porous body of the present invention have the following features in terms of increasing the growth rate of the metal-plated layer formed on the surface of the conductive layer:
(a) the nickel particles forming the conductive layer have an average particle diameter not less than 10 nm and not more than 300 nm; and
(b) congregation of the particles gives the conductive layer uninterrupted conductivity as a whole.
In the process of nickel deposition, finer particles called primary particles first deposit. Then these primary particles congregate and constitute a secondary particle. In the present invention, the terms xe2x80x9cparticle diameterxe2x80x9d and xe2x80x9caverage particle diameterxe2x80x9d are used to mean the diameter and the average diameter, respectively, of the secondary particles.
It is desirable that the conductive layer contain not more than 100 ppm titanium oxide derived from a reducing agent. The conductive layer may allow titanium oxide produced as a by-product during the reducing process of nickel to mingle with as the nucleus of the nickel-layer deposition. Provided that the titanium-oxide content in a conductive layer is 100 ppm or less, the titanium oxide has practically no effect on the electrical resistance of the conductive layer. More importantly, metal titanium has practically no opportunity to contaminate the conductive layer.
The metallic porous body of the present invention has a continuous metal-plated layer (nickel-plated layer, for example) formed on the surface of the framework of the conductive porous body of the present invention by electroplating, with the conductive porous body serving as the cathode.
The metallic porous body of the present invention produced by using the conductive porous body allows trace amounts of titanium oxide produced as a by-product during the reducing process to diffuse into the metal-plated layer as an impurity, although the amount is minimal, when the plastic core is removed by heat treatment, for example. As a result, the electrical resistance increases slightly in comparison with a pure elemental metal (elemental nickel, for example). Nevertheless, the increment is extremely small in comparison with the increase in electrical resistance by the diffusion of phosphorus from the above-mentioned conventional electroless nickel-plated layer formed by using sodium hypophosphite, and gives practically no effect on the total electrical resistance of the metallic porous body.
The present invention offers a conductive porous body that has the following features:
(a) Its conductivity can be obtained without using an expensive catalyst such as palladium.
(b) It has excellent conductivity, which has no tendency to decrease by deformation such as drawing or bending.
(c) It allows a metal-plated layer to grow quickly in electroplating. (d) It enables the production of an excellent metallic porous body that is superior in productivity and production efficiency and has low electrical resistance.
The present invention also offers a metallic porous body that has low electrical resistance because it is produced by using the foregoing conductive porous body and that enables the increase in charging and discharging efficiency when used as a battery plate, for example. The metallic porous body may be produced by removing the plastic porous body by heat treatment after the formation of the metal-plated layer.
The present invention also offers a battery plate that consists mainly of the metallic porous body of the present invention. The battery plate is superior in charging and discharging efficiency because of the use of the metallic porous body.